Citigroup announced on June 16 that more than 200,000 new credit cards had been issued to hacked customers. In some cases, customers had already closed their account or had received a new credit card, so they didn't need the Citi-initiated replacement.

Citigroup waited until June 3, more than three weeks after its discovery of the hack, to start sending out notification letters. However, the company insisted that it acted quickly to deal with the security problem

"From the moment Citi discovered the breach, we took immediate action to rectify the situation and protect any customers potentially at risk," Citi said in a written statement earlier this month.

There has been a spate of recent, high-profile security breaches. Video game maker Electronic Arts (EA) said Friday that hackers recently breached a server linked to a message board, stealing customer information.

Sony (SNE) was subjected to major hacks in April and May, affecting several of its gaming systems and potentially compromising tens of millions of credit card numbers.